Sen. Edward Kennedy, the longtime Massachusetts Democrat and head of the Kennedy family, has a malignant brain tumor, his doctors said Tuesday.

Doctors at Massachusetts General Hospital, who were investigating the cause of a seizure that Kennedy, 76, suffered at his Cape Cod compound on Saturday, said preliminary results from a biopsy of the brain had revealed that the senator has a malignant glioma in the left parietal lobe, the upper left part of his brain.

Dr. Lee H. Schwamm, the hospital's vice chairman of neurology, and Dr. Larry Ronan, Kennedy's primary-care physician at the hospital, said in a statement that "the usual course of treatment includes combinations of various forms of radiation and chemotherapy" and that "decisions regarding the best course of treatment for Sen. Kennedy will be determined after further testing and analysis."

News of the brain tumor surprised people in Washington, Massachusetts and beyond, generating reaction around the world, where Kennedy's family legacy and his 46 years in the Senate have made him a well-known figure.

Aside from an unsuccessful run for president in 1980, Kennedy has focused his energy on issues such as health care, education and civil rights. Despite his liberal ideology and occasional loud clashes on the Senate floor, Kennedy is held in high esteem by Republicans for his determination, understanding of issues and willingness to work in a bipartisan fashion.

"Sen. Kennedy enjoys great respect and admiration on this side of the aisle," said Sen. Mitch McConnell of Kentucky, the Republican leader. "He is indeed one of the most important figures to ever serve in this body in our history."

Doctors and people close to Kennedy said he will remain in the hospital for the next couple of days. The doctors said he is in overall good condition and remains in good spirits and full of energy. He has not had another seizure since he was hospitalized, they said.

"Right now, he's his normal self, except for the news that he's dealing with," said a close friend who spoke on condition of anonymity. "I bet he'll be back at the Cape sailing this weekend. I expect he'll go back to work" after the Memorial Day recess.

Senate Democrats and Republicans learned of Kennedy's condition as they were gathered for their weekly closed-door party luncheons, and members of both parties were visibly shaken by the news.

As he opened debate on the Iraq spending bill, Sen. Robert Byrd, D-W.Va., the only incumbent senator who has served longer than Kennedy, was distraught. "Ted, Ted, my dear friend, I love you and miss you," Byrd said in halting remarks on the floor.

Sen. John Kerry, the junior senator of Massachusetts, said on Tuesday in Washington that he had visited Kennedy over the weekend.

"He's in a fighting mood," Kerry said. "He is asking questions about what the choices are for him. He's deeply involved in making all the kinds of personal decisions that any of you would."

In Massachusetts, the impact of Kennedy's persona and political legacy is hard to overestimate.

"There'll never be another Ted Kennedy," said Paul S. Grogan, president and chief executive of the Boston Foundation, an organization that finances nonprofits involved in economic development and other state issues. "He's been such an outsized figure, so influential, so effective."

Grogan said that Kennedy had given Massachusetts a level of political prominence beyond what would normally be accorded a state of its size, and that he had helped engineer policies and financing mechanisms that benefited important sectors of the state, including universities and medical centers. Just one example, he said, was that Massachusetts institutions receive a large proportion of financing from the National Institutes of Health.

"He's single-handedly postponed the onset of Massachusetts' political decline, and whenever Sen. Kennedy is no longer active in politics, I think that is one of the things that is going to be incredibly shocking to people," Grogan said. "His vigor, his vitality and his longevity ... kind of encouraged us all to believe that he's immortal, and we've gotten used to the idea that he's going to be around forever. But this is a reminder that he's not."

Colleagues, family react

Responses to the news that Sen. Edward Kennedy, D-Mass., has a malignant brain tumor:

"Laura and I are concerned to learn of our friend Sen. Kennedy's diagnosis. Ted Kennedy is a man of tremendous courage, remarkable strength and powerful spirit. Our thoughts are with Sen. Kennedy and his family during this difficult period." - President Bush

"He's comforted by the fact that his dad is such a fighter, and if anyone can get through something as challenging as this, it would be his father." - Robin Costello, spokeswoman for Rep. Patrick Kennedy, D-R.I.

"Maria (Shriver, Kennedy's niece) and I are thankful for everyone's thoughts and prayers today and over the past several days. ... Teddy is an incredibly courageous and tenacious man who will tackle this with the same determination with which he approaches everything in life." - Gov. Arnold Schwarzenegger

"I have confidence though because Sen. Kennedy has been a fighter all his life. ... I know that that fighting spirit will hold him in good stead in the challenge that he faces now." - House Speaker Nancy Pelosi, D-San Francisco

"I am so deeply saddened I have lost the words." - Sen. John Warner, R-Va.

"I have said on numerous occasions, I have described Ted Kennedy as the last lion in the Senate, and I have held that view because he remains the single most effective member of the Senate if you want to get results." - Sen. John McCain, R-Ariz.

"He fights for what he thinks is right. And we want to make sure that he's fighting this illness. And it's our job now to support him in the way that he has supported us for so many years." - Sen. Barack Obama, D-Ill.

"Ted Kennedy's courage and resolve are unmatched, and they have made him one of the greatest legislators in Senate history. Our thoughts are with him and Vicki, and we are praying for a quick and full recovery." - Sen. Hillary Rodham Clinton, D-N.Y.

"I'm having a hard time remembering a day in my 34 years here I've felt this sadly." - Sen. Patrick Leahy, D-Vt.